The invention relates to a piezoelectric measuring element consisting of at least two piezoelectric crystals showing a transversal effect for the measurement of axially acting forces and/or pressures as well as to a pressure or force sensor comprising said element.
Piezoelectric crystals are widely used in sensors for the measurement of forces, pressures, acceleration, elongations, and moments or combinations thereof. For this purpose, for example crystals showing a transversal effect are cut into thin plates or bars. If used in measuring technology, such thin plates or strips are for example exposed to an axially acting force, i.e. a force that acts on two opposite end faces of a plate or a strip. If a crystal with transversal effect is concerned, an electric charge accumulates under the force on the two major lateral surfaces of the plates or strips. On electrically conductive layers, so-called electrodes, applied to these two lateral surfaces and being not in electrical contact with each other these charges are collected and transmitted by means of the respective device within the sensor. Eventually, a suitable unit detects the signal generated by these charges from the sensor whereby information with respect to the force can be obtained. In this regard, the polarization describes the direction of each electrode negatively charged under the acting force with respect to each positively charged electrode.
Measuring elements of this type are sufficiently known and are for example described in the publication published 1988 “PIEZOXIDE (PXE)” BY VALVO, a division of Phillips GmbH, Essen, or 2002 in “Piezielectric Sensorics”, by G. Gautschi, published by Springer Verlag.
It is critical for the quality of a sensor that each of the crystals is incorporated perpendicularly and centered parallel to the sensor axis. A slightly inclined position will result in an incorrect measurement or in breakage of the crystal under the action of the forces to be applied later.
The sensitivity of this sensor is proportional to the relationship between the bar length to the bar thickness of the crystal. In this respect, the length of the bar is defined by the distance between the end faces leading the force into the crystal and the thickness through the distance between the end faces where the charges accumulate under a force. The thinner a crystal is the higher is therefore its sensitivity at an identical length. On the other hand, the maximum stability under load of thin plates is smaller than that of thick plates because they break more easily.
In FIGS. 1 to 4, the direction of the arrows designated by the numeral 4 always shows the direction of the polarization 4 of a piezoelectric crystal from minus to plus. The faces of the crystals acquiring a positive (+) or a negative (−) charge under a force are coated with electrodes. Those electrodes acquiring a positive charge under a force are always shown hatched with a solid outline; those electrodes acquiring a negative charge under a force are always shown dotted with a dotted outline. The directions of polarization and thus also positive and negative charges were chosen voluntarily for the representation and can be reversed or exchanged, respectively, in each of the Figures.
FIG. 1 shows the representation of a known simple piezoelectric measuring element 1 with transversal effect. The end face 3 which is the major one in this embodiment is advantageous for easy centering during the incorporation into the sensor housing. Since the sensitivity is proportional to the mean thickness d of the crystal it is not very high.
In order to increase the sensitivity, the crystal 1 can be made thinner. FIG. 2 shows such a crystal 1 which is also known in the form of a thin plate. The higher sensitivity due to the small thickness d, however, reduces the simplicity of centering during mounting into a sensor. Furthermore, the maximum stability under load is limited since this thin crystal 1 breaks easily.
To create a high sensitivity and simultaneously a high stability under load several thin crystals can be fitted into the same sensor. The maximum stability under load of the sensor thus multiplies by the number of crystals introduced while the sensitivity is the same as if only a single crystal having the same thickness was used. Taking into account the force shunt, however, the sensitivity will be higher since the stiffness of the sum of the crystals is increased by the number of crystals.
A known arrangement consists of three crystals having the cross section of circle segments or rectangles. These are arranged in a spherical sensor in a way that a triangle is provided in the centre. Such arrangements are for example known from CH 392 103.
FIGS. 3 (a) and (b) show a known measuring element 2 consisting of three crystals 1 having the same lengths. They have a cross sectional area 5 corresponding to a circle segment and are arranged such that they can be placed into a spherical sensor housing while a triangular area 6 remains in the centre. Under a force, the electrodes on the surfaces facing the interior all collect the same charge which in this embodiment is received by an electrically conductive spiral spring 7 represented in FIG. 3b. The charge of the electrodes on the outer surface of the crystal is received by the casing of the spherical sensor housing onto which the crystals 1 are pressed by the spiral spring 7. A measuring element 2 of this type is characterized by its high stability under load and high sensitivity. Fabrication and mounting and also the centering, however, remain very complex.
Another known arrangement uses four thin crystal plates spaced in a side by side manner and having the same length.
The known embodiment of a measuring element 2 represented in FIGS. 4 (a) and (b) shows the same advantages and disadvantages as the embodiment of FIG. 3. In this case for example four crystals 1 having the same height are spaced in a side by side manner each having an opposite polarization direction 4. For the withdrawal of charge, a spring 8 in the form of a corrugated sheet is placed between each two of the adjacent crystals 1 as shown in FIG. 4b. Such a spring 8 always ensures a good contact for charge transmission.
Mounting and particularly centering of the crystals, however, is even more difficult in these multiple crystal arrangements.
During mounting care must be taken in these two cases that both electrodes of each crystal are perfectly connected with the lines within the sensor.